1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting alignment marks which are used in measuring a reference position to which a mask pattern and a semiconductor wafer pattern are aligned during exposure.
2. Description of the Background Art
In an exposure apparatus which is used to obtain a resist pattern, to have an excellent mask-wafer alignment accuracy is as important as to have an excellent resolution. Briefly speaking, "alignment" as termed in the semiconductor industry refers to aligning a mask pattern to a previously formed wafer pattern. In actual procedures, "alignment" involves detecting the alignment marks, and aligning mask marks to the alignment marks which are previously formed on a semiconductor wafer at predetermined positions.
FIG. 1 shows an example of alignment marks. In FIG. 1, alignment marks are diffraction gratings 6 which are formed on scribe lines 2 scribed on a semiconductor wafer 1. Lines of the diffraction gratings 6 extending in a direction X are a diffraction grating array 3Y, hereinafter referred to as "a Y-mark." The Y-mark is used to detect the position of an alignment mark which extends in a direction Y (described later). Lining in the direction X, the diffraction gratings 6 form a diffraction grating array 3X which is used in detecting the position of an alignment mark which elongates in the direction X. The diffraction grating array 3X will be hereinafter referred to as "an X-mark."
FIGS. 2A and 2B are explanatory diagrams for explaining a conventional alignment method. A first step of the conventional alignment method is to split a laser beam into two laser beams, one for illuminating an X-mark and the other for illuminating a Y-mark. The illumination laser beams 4 thus produced are illuminated onto the semiconductor wafer 1 at preselected laser beam illumination positions through a projection optical system. As shown in FIG. 2B, the laser beam 4 for illuminating the Y-mark irradiates the semiconductor wafer 1 in the shape of a strip which extends in the direction X. The laser beam 4 for illuminating the X-mark, on the other hand, falls on the semiconductor wafer 1 in the shape of a strip extending in the direction Y (not shown). The semiconductor wafer 1 is moved in the direction Y to scan the Y-mark illumination laser beam 4 on the semiconductor wafer 1 (FIG. 2B), during which the signal intensity of a diffractive light beam from the semiconductor wafer 1 is successively measured. The signal intensity of the diffraction light beam varies a the semiconductor wafer 1 slides. More precisely, when a Y-mark 3Y comes to the laser beam illumination position, a diffraction light beam is produced by the diffraction gratings 6 of the Y-mark 3Y, thereby largely changing the signal intensity under measurement. Hence, a Y-coordinate of the Y-mark 3Y is detected by measuring a change in the signal intensity of the diffraction light beam. An X-coordinate of an X-mark 3X is detected in a similar manner: The semiconductor wafer 1 is moved in the direction X to scan the X-mark illumination laser beam 4 on the semiconductor wafer 1 and the signal intensity measurement is carried out during the scanning. Thus, it is possible to detect the X- and the Y-direction alignment marks of the semiconductor wafer 1.
However, theory and reality do not always agree. In an actual structure of the semiconductor wafer the diffraction gratings 6 forming the X- and the Y-marks 3X and 3Y do not have ideal configurations as those shown in FIG. 2A due to the existence of an overlying element such as a metal film 5 as shown in FIG. 3. Metal particles deposited arrisways the surface of the semiconductor wafer 1 are especially problematic because such deposition causes the diffractive gratings 6 of the X- and the Y-marks 3X and 3Y to have asymmetric configurations. This makes it impossible to produce desirable diffraction light beams. Since desired diffraction light beams are not obtainable, chances are that the alignment marks of the semiconductor wafer 1 cannot be accurately detected.
If the diffractive gratings 6 each have an ideal configuration as shown in FIG. 4, the intensity of a diffraction light beam becomes strongest (intensity peak P) when the diffractive gratings b are located at the laser beam illumination position. Hence, it is possible to detect the positions of the diffractive gratings 6 by finding where the intensity peaks P are observed. The opposite case is illustrated in FIG. 5. In FIG. 5, the diffractive grating 6 has an asymmetrical shape due to a metal film 7 deposited thereon. Due to the asymmetrical shape of the diffractive grating 6, the intensity of a diffraction light beam grows extremely strong for more than once, exactly three times in the example of FIG. 5 (intensity peaks P1 to P3), in the vicinity of the diffractive grating 6. As a result, it becomes very difficult to detect the precise position of the diffractive grating 6.
Thus, in the conventional alignment method, a film formed on the semiconductor wafer 1 deteriorates a detection accuracy of detecting the X- and the Y-marks 3X and 3Y (i.e., diffraction grating arrays) which function as alignment marks.